If a vehicle accelerates on a slippery road surface, the force that is supposed to be transmitted to the undersurface by the tires may be greater than the cohesive friction between the tires and the subsurface. This results in the tires spinning, and a lesser friction, sliding friction, develops between the tire and the subsurface. As that occurs, the vehicle may behave in a manner unexpected by the driver; for example, it may not accelerate in the accustomed manner.
This detracts from the driving comfort and safety.
A great number of methods are known in automotive engineering that prevent the drive wheels from spinning in motor vehicles, in particular in automobiles.
To prevent the drive wheels from spinning, the drive wheels may be braked by a conventional traction control, or the driver operates the accelerator pedal less so that less power is transmitted from the drive engine to the drive wheels.
Both methods have the effect that the torque acting on the wheels is reduced, and thus the power that is to be transmitted from the drive wheels to the street is less than the friction between the tires and the street. The result is that cohesive friction again prevails between the wheels and undersurface, and the wheels no longer spin.
DE 696 04 654 T2 shows a control method for automated shift transmissions with a device for avoiding tire slip. It is proposed there that the state of little adhesion of the tires be detected, and that a minimal change of the engine torque conveyed to the wheels be ensured. In so doing the following principles should be parameterized: reducing the engine torque before shifting gears, disengaging the clutch, selecting another gear, engaging the clutch after the gear is selected, restoring the engine torque after shifting gears, and engagement while the vehicle is starting up.
In addition, it is proposed that the shift to second gear be effected automatically while the vehicle is standing still.
DE 698 08 249 reveals the detection of a slippery undersurface, based substantially on analyzing the gradient of a representative parameter for the speed of the driven wheels. This analysis enables automatic activation of a specific mode for controlling the transmission ratios, which is matched to the adhesion of the drive wheels.
In both methods, the driver is confronted with the unexpected behavior of the vehicle right in the exceptional situation in which one or more drive wheels are spinning. Hence the shifting process, in particular the disengagement of the gear shift system, interrupts the transmission of torque to the wheels, and thus an unwanted driving situation is forced on the driver.